To shift a transmission, a shifting device is provided inside the transmission in a vehicle to help engage and disengage individual gears. For this purpose, components of the shifting device can be actuated manually by the driver of the vehicle or shift elements or actuators that can be operated by auxiliary means, for example hydraulic fluid, are provided and these act upon the components of the shifting device, as necessary, to shift the gears.
From EP 0 541 035 B1, a shifting device for multiple-gear, range-change transmissions is known, which comprises valves and shift cylinders in which there are pistons actuated by a control fluid. Shift rods are attached to the shift cylinders, and act, via shift elements, on components of the transmission to form a torque-transmitting connection. A shift rod actuates the components associated with the speed range.
In this case, it is disadvantageous that the shift rods have substantial mass which contributes to the overall mass of the transmission with adverse effect and which has to be accelerated and braked each time a shifting operation is carried out.
From DE 197 56 639 A1, a shift device is known in which shift cylinders accommodating pistons displaced by a fluid are provided. These are integrally connected with shift elements, namely shift forks. The piston connected to a shift fork can be moved axially in two directions along a guide rail with which the two-part cylinder housing is solidly connected, whereby the shift element too is moved.
In the shifting device known from DE 197 56 639 A1, it is advantageous that, owing to its construction as a compact unit, connecting elements between the shift elements consisting of shift forks or shift rockers and the shift cylinders or shift pistons are not needed so that the reaction times, between the command that triggers a gearshift and the movement of the shift elements that bring it about, can be made considerably shorter due to the absence of the heavy shift rods that would otherwise have to be moved.
A disadvantage of such a shifting device, however, is the integrated design of the shift piston and shift element which, in particular, makes maintenance and replacement of the shifting device more difficult. Likewise, integration or adaptation of such an arrangement in an existing transmission is hardly possible. In some types of transmissions, it can also be disadvantageous for such an arrangement that the piston can only move axially along its guide rail. With a design of that type, swiveling or tilting of a shift element, which is desirable in some circumstances, is not possible.
A further problem of the known systems is that the shift times are relatively long, which can be attributed to the long hydraulic line connections to the shift elements. In addition, such lines often have to be passed through restricted spaces and bent through quite sharp angles and this results in comparatively high line resistances and hence long shift times.
Against this background, the purpose of the present invention is to provide a shifting device of the type outlined, which enables short shift times, which is of compact structure and which is repair- and maintenance-friendly.